CT scans are known to pose a greater risk than ordinary X-rays such as mammograms, but the new research indicates they might be more hazardous than was previously thought.

Results of the US study published in the journal Archives of Internal Medicine showed that CT radiation doses were generally higher than typically reported.

For individual types of scan, there was an average 13-fold variation between the highest and lowest doses experienced by patients.

“If a physician sent a patient for a particular CT procedure, the dose that patient would have received varied by this much,” said study leader Professor Rebecca Smith-Bindman, from the University of California at San Francisco.

“The risk associated with obtaining a CT is routinely quoted as around one in 1,000 patients who undergo CT will get cancer. In our study, the risk of getting cancer in certain groups of patients for certain kinds of scans was as high as one in 80.”

During 2008/9 in England there were a total of almost 36 million scans, x-rays and tests involving ultrasound, magnetic and radio waves, and radiation, of which 3.3 million were CT scans